The present invention relates to a YIG (Yttrium Iron Garnet) film microwave apparatus having means for applying a DC biasing magnetic field to a microwave device employing a ferrimagnetic YIG film resonator.
There has been proposed a microwave apparatus, such as a microwave filter or a microwave oscillator, utilizing the ferrimagnetic resonance of a ferrimagnetic resonator constructed by forming a film of a ferrimagnetic YIG film over a nonmagnetic GGG (Gadolinium Gallium Garnet) substrate by a liquid-phase epitaxial growth process (hereinafter abbreviated to "LPE process") and selectively etching the YIG film by a photolithographic process in a desired shape such as a circular or a rectangular shape.
Such a microwave device is capable of being used with microstrip lines as transmission lines electromagnetically coupled to the YIG thin film for a microwave integrated circuit and facilitates the hybrid connection of one microwave integrated circuit and another microwave integrated circuit. Furthermore, the LPE process and the photolithographic process enable the mass production of the microwave device utilizing the magnetic resonance of a YIG film. The microwave device utilizing the magnetic resonance of a YIG film has many practical advantages over the conventional microwave device employing a YIG sphere.
However, since the ferrimagnetic resonance frequency of the YIG film is greatly dependent on temperature, the microwave apparatus employing a YIG film has inferior temperature characteristics, which is a significant problem in the practical application of the microwave apparatus.
This problem will be described more specifically hereinafter.
Suppose that a YIG film is disposed in a gap of a magnetic circuit so that a DC magnetic field is applied perpendicularly to the film surface thereof and the contribution of an anisotropy field is negligible. Then, the ferrimagnetic resonance frequency of the YIG film can be expressed on the basis of the Kittel's formula: EQU f(T)=.gamma.{Hg(T)-Nz.sup.Y .multidot.4.pi.Ms.sup.Y (T)}
where .gamma. is gyromagnetic ratio (.gamma.=2.8 MHz/Oe), Hg is DC gap magnetic field, Nz.sup.Y is the demagnetization factor of the YIG film calculated on the basis of the magnetostatic mode theory, and 4.pi.Ms.sup.Y is the saturation magnetization of the YIG film. Since Hg and 4.pi.Ms.sup.Y are functions of temperature T, resonance frequency f is a function of temperature T. Concretely, in the perpendicular resonance of a YIG disk having an aspect ratio (thickness/diameter) of 0.01, Nz.sup.Y =0.9774 and if the biasing magnetic field intensity Hg is fixed regardless of temperature, 4.pi.Ms.sup.Y is 1916 G at -20.degree. C. and 1622 G at +60.degree. C. Thus, the deviation of the resonance frequency f in this temperature range is as large as 823 MHz.
Such temperature-dependent deviation of the resonance frequency of a YIG microwave apparatus is avoidable by placing the YIG magnetic resonator in a thermostatic chamber to keep the YIG magnetic resonator at a fixed temperature or by varying the magnetic field intensity by means of an electromagnet according to temperature deviation so that the resonance frequency is maintained at a fixed level. However, these methods require external energy supply and additional control means such as means for controlling electric current and hence a complex constitution.